1. Field of the Invention
This invention generally relates to an auto control circuit for a frequency converter, and more particularly to an auto control circuit for operating an equipment in an abnormal voltage for a predetermined time period.
2. Description of Related Art
In the modern industrial era, the operation of the production line is a significant indication of the economic strength of a country. Hence, whether the smooth operation of equipments in the factories is an important issue. The core of the equipment in the factories is the driven motor. The frequency converter is widely used in the motors. Hence, the characteristics of the control circuit of the frequency converter are very important.
FIG. 1 illustrates a traditional control circuit of a frequency converter. Referring to FIG. 1, the traditional frequency converter includes a switch circuit 11, a starter circuit 12 and a starter relay 13. The switch circuit 11 can be turned on an off via starter circuit 12. The switch circuit 11 is connected to the starter relay 13 and the starter relay 13 controls the frequency converter 14 of the equipment 15.
When the voltage of the power supply to the traditional frequency converter is below a predetermined voltage level, the starter relay 13 will be turned off according to the IEEE446 standard. Hence, there is no output from the frequency converter 14 so that the equipment 15 stops working.
FIG. 2 is the characteristic curve during the operation of the frequency converter. Referring to FIG. 2, at time T1, the voltage level of the power supply starts to decrease. According to the IEEE446 standard, when the voltage level of the power supply is below 85% of the working voltage, the frequency converter stops output and thereby stopping the operation of the equipment. At time T5, the equipment completely stops. If the equipment stops due to the transient voltage drop, it would affect the production throughput.